What is canine influenza? (DOG FLU) Answer: Canine influenza is a highly contagious respiratory infection of dogs caused by a novel influenza virus that was first discovered in 2004. We do not use the general term “dog flu” because it could refer to any flu-like illness in dogs due to various causes. Rather, canine influenza is a specific disease caused by a particular subtype, H3N8, of the influenza A virus.